Photosynthesis of Vascular Plants
417
presented by Beyschlag et al. (1990) for the photosynthetic parameters and
by Barnes et al. (1990) for the structural input data clearly revealed (1) that
single-leaf photosynthetic behavior is not necessarily representative of the
integrated canopy behavior and (2) that in these dense grass stands, the
structural canopy properties are much more important for determining the
balance of light competition than the functional characteristics, such as
photosynthetic capacity. The following two case studies will illustrate how
effectively models of this type can be employed as a tool for analysis of
aboveground competition.
20.4.2 Case Study: Symmetric Competition
Plants of similar stature can compete for light, and each plant exerts some
shading effect on its neighbors, usually in proportion to its size. Such cases
of symmetrical competition may involve mutual shading at several levels in
the canopy rather than one plant simply overtopping another.
In field and glasshouse experiments, mixed canopies of wheat and wild
oat were given supplemental UV-B radiation and the relative competitive
status of wheat increased (Barnes et al. 1988). This was evaluated from
aboveground biomass comparisons between mono cultures and mixtures
using relative crowding coefficients of de Wit (1960). Associated with the
shift in the competitive balance under UV-B treatment were differential
effects of UV-B on the growth form of the two species: leaf insertion heights
and leaf blade lengths were reduced to a greater extent in wild oat than in
wheat. In monocultures of these species, these growth form changes had no
measurable bearing on the total production of the plant stands (Barnes et al.
1988). However, in mixtures, these changes in individual plant morphology
resulted in shifts in foliage height distribution such that wheat tended to have
its foliage somewhat higher in the canopy than did wild oat under UV-B enhancement. Assuming that carbon gain was related to biomass production,
the question was whether these structural changes in the two species could
quantitatively account for the changes in biomass of the two competitors.
Figures 20.4 and 20.5 show the results of model calculations using the
multi species version of the canopy photosynthesis model which had been
parameterized either with the structural and photosynthetic data of control
canopy mixtures without enhanced UV-B radiation or mixed canopies given
the UV-B enhancement treatment. The results clearly demonstrate that a
comparatively small but differential change in the structural properties of
the two competing species can lead to changes in canopy net photosynthesis
sufficient to explain the observed shift in the competitive balance. Barnes
et al. (1990) further showed that the changes in leaf area were largely
responsible for the effects, whereas changes in leaf angles had a negligible
effect.
417
presented by Beyschlag et al. (1990) for the photosynthetic parameters and
by Barnes et al. (1990) for the structural input data clearly revealed (1) that
single-leaf photosynthetic behavior is not necessarily representative of the
integrated canopy behavior and (2) that in these dense grass stands, the
structural canopy properties are much more important for determining the
balance of light competition than the functional characteristics, such as
photosynthetic capacity. The following two case studies will illustrate how
effectively models of this type can be employed as a tool for analysis of
aboveground competition.
20.4.2 Case Study: Symmetric Competition
Plants of similar stature can compete for light, and each plant exerts some
shading effect on its neighbors, usually in proportion to its size. Such cases
of symmetrical competition may involve mutual shading at several levels in
the canopy rather than one plant simply overtopping another.
In field and glasshouse experiments, mixed canopies of wheat and wild
oat were given supplemental UV-B radiation and the relative competitive
status of wheat increased (Barnes et al. 1988). This was evaluated from
aboveground biomass comparisons between mono cultures and mixtures
using relative crowding coefficients of de Wit (1960). Associated with the
shift in the competitive balance under UV-B treatment were differential
effects of UV-B on the growth form of the two species: leaf insertion heights
and leaf blade lengths were reduced to a greater extent in wild oat than in
wheat. In monocultures of these species, these growth form changes had no
measurable bearing on the total production of the plant stands (Barnes et al.
1988). However, in mixtures, these changes in individual plant morphology
resulted in shifts in foliage height distribution such that wheat tended to have
its foliage somewhat higher in the canopy than did wild oat under UV-B enhancement. Assuming that carbon gain was related to biomass production,
the question was whether these structural changes in the two species could
quantitatively account for the changes in biomass of the two competitors.
Figures 20.4 and 20.5 show the results of model calculations using the
multi species version of the canopy photosynthesis model which had been
parameterized either with the structural and photosynthetic data of control
canopy mixtures without enhanced UV-B radiation or mixed canopies given
the UV-B enhancement treatment. The results clearly demonstrate that a
comparatively small but differential change in the structural properties of
the two competing species can lead to changes in canopy net photosynthesis
sufficient to explain the observed shift in the competitive balance. Barnes
et al. (1990) further showed that the changes in leaf area were largely
responsible for the effects, whereas changes in leaf angles had a negligible
effect.